It has been well known that polyolefins such as crystalline polypropylene are obtained by polymerizing olefins in the presence of so-called Ziegler-Natta catalyst which comprises a compound of a transition metal of Group IV to Group VI in the periodic table and an organometallic compound of a metal of Group I to Group III of the periodic table. Recently, there have been made studies on a process in which crystalline polyolefins of high stereoregularity can be obtained with high polymerization activity using such catalysts as mentioned above, as described in, for example, Japanese Patent Laid-Open Publications No. 209207/1986, No. 104810/1987, No. 104811/1987, No. 104812/1987, No. 104813/1987, No. 311106/1989, No. 318011/1989 and No. 166104/1990.
Such a crystalline polypropylene having a high stereoregularity is high in rigidity and usually has a high heat distortion temperature, a high melting point and a high crystallization temperature, and hence it shows excellent properties such as high heat resistance, high crystallization speed and high transparency. Accordingly, the crystalline polypropylene has been applied to various uses such as containers and films.
Though, a film made from the above-mentioned crystalline polypropylene has excellent water-vapor barrier properties as compared to a film comprising polyamide or polyester, it is not always sufficient in the water-vapor barrier properties to apply for the use in which an extremely high water-vapor barrier property is required, for example, a film for packaging a cigarette. Therefore, for such use that an extremely high water-vapor barrier property is required, the so-called K-OP film of which polyvinylidene chloride (PVDC) is coated on the surface of a biaxially oriented polypropylene film (OPP film) is usually used. However, the K-OP film has such problems that chlorine gas generates when incinerating, since it contains the PVDC. Moreover, the film cannot be recycled by means of a re-extrusion, since the PVDC has a poor compatibility with polypropylene.
For solving the above-mentioned problems, Japanese Patent Publication 3-47177 (1991) discloses a polypropylene stretched film comprising polypropylene and a petroleum resin having no polar group or a terpene resin having no polar group, and having a glass transition temperature (Tg) of 10.degree. to 80.degree. C. However, the water-vapor barrier properties of this film are inferior to the K-OP film, so that it is difficult to apply for the use requiring an extremely high water-vapor barrier property.
The present inventors have made intensive and extensive study to solve the above-mentioned problems, and a result, they have found that a stretched film formed from a polypropylene resin composition comprising a specific crystalline polypropylene having a high stereoregularity and a terpene resin having no polar group and/or a petroleum resin having no polar group has excellent water-vapor barrier properties.
In recent years, a press-through pack packaging (hereinafter sometimes referred to as "PTP packaging") is well used for packaging drags. The PTP packaging is performed by forming a plurality of recess portions on a plastic sheet by thermoforming, loading a tablet or a capsule into the recess portion, and sealing it with aluminum foil. For such a plastic sheet for the PTP packaging, a rigid vinylchloride resin sheet is conventionally used. However, for a packaging for drugs which readly change quality thereof by humidity, such as antibiotics, the rigid vinylchloride resin sheet are not always sufficient in the water-vapor barrier properties. Therefore, for uses requiring a higher water-vapor barrier properties, a laminate sheet coated with PVC on the surface of a rigid vinylchloride resin sheet is used. However, there is such a problem that the laminate sheet is expensive. Further, there is such a problem that the rigid vinyl chloride resin and PVDC generate chlorine gas when incinerating.
On the other hand, a sheet made of conventional polypropylene has no problem in the generation of a chlorine gas when incinerating. However, it has problems in transparency and thermoformability, and therefore, the sheet is seldom used for the PTP packaging.
For improving the transparency of the conventional polypropylene sheet, there is known a method in which a nucleating agent such as benzylidene sorbitol is added to the polypropylene. However, this method has such a problem that the thermoformability of the sheet becomes poor when the sheet is applied to PTP packaging. Further, for improving the thermoformability of the conventional polypropylene sheet, there is known a method in which polyethylene is blended to the polypropylene. However, this method has such a problem that the transparency of the sheet is extremely lowered.
The present inventors have made intensive and extensive study to solve the above-mentioned problems. As a result, it has been found that a sheet formed from a polypropylene resin composition comprising a specific crystalline polypropylene having a high stereoregularity and a hydrogenated petroleum resin is excellent in water-vapor barrier properties, as well as rigidity, and transparency, moreover, thermoformability. Based on these findings, the present invention has been accomplished.